In the preferred embodiment, the present invention is used in a data display system which employs a laser beam to thermally write on a high resolution liquid crystal light valve. In this system, two galvanometers are used to rotate two mirrors which reflect the laser beam onto the liquid crystal at a specified target position. The positioning system for controlling where the laser writes on the liquid crystal, such as the one disclosed herein, requires a very precise servo for controlling a galvanometer or other motor, and an equally precise position senor.
Various systems use light sources and light responsive devices for registration of data and data display. These systems incorporate finally focused light beams or laser beams that are employed for scanning and addressing light responsive surfaces. The scanning pattern can be along a single dimension, or over two dimensions of an X-Y plane, for example. When the beam is moved relative to the light receiving surface, it is generally necessary to determine accurately the position of the moving beam.
For example, in liquid crystal display technology, a scanning laser beam is used to thermally write on a high resolution liquid crystal light valve. Such displays can be selectively erased at a writing speed determined in part by the scanning speed of the beam. In another application, laser beams are used to scan photoresist for forming circuit board wiring configurations. In these types of applications, it is necessary to detect precisely the position of the beam.
Electromechanical galvanometer scanner systems have been used in the past to scan the laser beam light beam. The galvanometer systems incorporate a mirror for reflecting the laser beam in combination with a galvanometer for rotating the mirror. In one implementation, the position of the galvanometer shaft and the mounted galvanometer mirror can be determined by using a capacitance-based sensor, which consists of four electrodes surrounding an electrically grounded rotor. Movement of the galvanometer rotor produces differential currents in opposing electrode pairs, thereby generating a signal that is proportional to position. However, this approach does not take into account the flexing of the mirror or the shaft, the mechanical motion of the shaft axis, or any distortions which may be introduced by optical elements included in the galvanometer assembly. In addition, the speed of response to the galvanometer rotor movement is limited because of the added mass.
Another approach is to use an interferometric method wherein a coherent laser beam is reflected from a galvanometer mirror. The reflected light is modulated as the galvanometer mirror is moved. The spatial frequency of a grating determines the positional resolution. However, the disadvantage of interferometric methods is that they require coherent light, and therefore are expensive. In addition, such systems have less reliability than those using simple incoherent sources such as incandescent lamps.
Still another approach is to support a first grating on the rotatable member and a second grating mounted so that it is spaced from the first grating. Each grating includes transparent and opaque regions. Light is projected through the gratings and a photodetector senses the light which passes through the gratings. As the gratings move with respect to one another, a moire pattern is formed with the transmitted light increasing and decreasing in intensity depending upon whether the transparent and opaque regions overlap or are interleaved. The frequency of the output signal is a measure of the relative velocity and the number of cycles is a measure of the relative distance moved. A major problem with the prior art is that the gratings or other elements are generally mounted for movement with the moving or rotating member. This adds mass and increases the inertia of the movable member inhibiting its performance.
It is a general object of the present invention to provide a position sensor for sensing the position of a rotatable element which employs light reflected from the movable element rather than including an additional element mounted for rotation therewith.
Another problem with prior art systems is that the inherent instabilities of galvanometer servo systems have limited their accuracy and speed of operation. The present invention overcomes some of these difficulties by using a servo which incorporates a digital processor to determine the galvanometer's drive current and by using a servo configured to automatically minimize positioning error.